A TMR (tunneling magneto-resistance) sensor (MTJ device) whose free layer is limited to FeCo (bcc) will have a large TMR ratio but other magnetic properties, such as Hc (coercive field), Hk (anisotropy field), and lambda (magnetostriction), will have values that fall well outside the usable range. In most current TMR manufacturing processes it is normal practice to deposit NiFe (fcc) as an additional component of the free layer in order to achieve a softer free layer. However, use of FeCo/NiFe as the free layer will also substantially reduce the TMR ratio (dR/R) compared to what may be achieved with a FeCo only free layer.
The present invention discloses a structure, and method for its manufacture, which makes it possible to achieve the high TMR ratio associated with a FeCo free layer without suffering an attendant degradation of other magnetic characteristics.
A typical MR memory cell of the prior art is illustrated in FIG. 1. Seen there are magnetic pinning layer 11 (normally an antiferromagnetic layer of a material such as IrMn or MnPt), magnetically pinned layer 12 (either a ferromagnetic layer or, more commonly, a synthetic antiferromagnetic trilayer), transition layer 13 (either copper for a GMR device or a thin insulating layer for a TMR device), CoFe layer 14, NiFe layer 15 (which, together with layer 14, makes up the free layer), and capping layer 16.
A routine search of the prior art was performed with the following references of interest being found:
U.S. patent application 2005/0243477 (Gill) discloses a thin layer of Fe (on the order of several Angstroms) may be added to the free layer adjacent to the coupling layer to assure antiparallel coupling. U.S. Pat. No. 7,116,529 (Yoshikawa et al) is an example of many patents that disclose a free layer comprising FeCo/NiFe. U.S. Pat. No. 7,046,489 (Kamiguchi et al) shows a free layer of Fe/CoFe/Fe or Fe/NiFe/Fe.